The proliferation of modern wireless communications devices, such as cell phones, smart phones, and tablet devices, has seen an attendant rise in demand for high volume multimedia data capabilities for large populations of user equipment (UE) or mobile stations. To support this ever growing demand, future radio access (FRA) schemes will need to provide significant gains in capacity and quality of user experience (QoE) over conventional wireless access systems. Conventional wireless access technologies include a variety of transmission techniques such as time division multiple access (TDMA), code division multiple access (CDMA), frequency division multiple access (FDMA), orthogonal frequency division multiple access (OFDMA), and single carrier FDMA (SC_FDMA) as well as others, and are defined in various specifications including Long Term Evolution (LTE) and LTE-Advanced (LTE-A) being developed by the third generation partnership project (3GPP), the 802.11 and 802.16 family of wireless broadband standards maintained by the Institute of Electric and Electronic Engineers (IEEE), and WiMAX, an implementation of the IEEE 802.11 standard from the WiMAX Forum. In general these conventional systems provide transmission to many users by creating orthogonal concurrent channels separated by multiplexing with time, frequency, coding, and/or space. Servicing multiple users with separate orthogonal channels permits elimination of inter-user interference between orthogonal channels and results in relatively simple receiver designs. To obtain increased spectral efficiency, FRA systems are being proposed that will take full advantage of these conventional orthogonal technologies such as OFDMA and will obtain performance improvements by incorporating new technologies, such as non-orthogonal multiple access (NOMA), into conventional systems.
NOMA techniques superimpose symbols being transmitted to different users within the same time-frequency-spatial radio resources and exploit differences in channel gains and transmission power to separate user signals at the various UE. With NOMA, users having different signal to noise ratios are grouped together, typically with a “near user” having a stronger link to the base station, and one or more “far users” having weaker links. As used herein the term base station refers to an access node or other entity in the wireless network used to transmit signals to a plurality of UE or mobile devices. For example a common type of base station used in conventional systems is the enhanced Node-B (eNode-B) used in LTE systems. Users that have been grouped together to receive a non-orthogonal data signal are referred to herein as a superposition group. Data symbols for each user in the superposition group are then superimposed or superposed on a single radio resource, or resource element, and transmitted with different powers. A resource element (RE) is the smallest usable portion of the radio spectrum and has units of time and frequency. In multi-input multi-output (MIMO) transmission a RE may also occupy one spatial layer. Symbols belonging to near users and far users are superposed within the same RE using different transmit power weights. To differentiate between symbols that have been transmitted orthogonally from symbols that have been transmitted non-orthogonally the term superposed symbol is introduced to refer to symbols that are transmitted non-orthogonally, i.e. overlapped within a single time/frequency/space RE. Thus, symbols or data symbols that have been superposed within a single RE for non-orthogonal transmission are referred to herein using the term superposed symbols. A successive interference cancellation (SIC) type receiver can be used to successively detect/decode and cancel signals of other higher power users whose symbols or data overlap and interfere with the desired data. SIC removes these interfering symbols and reveals the data, or superposed symbols, belonging to the receiving UE. Higher power users are users whose superposed symbols are transmitted at a higher power within the same non-orthogonal data signal and are also referred to herein as interfering users. For example in a set of users grouped together for NOMA transmission, i.e. in a superposition group, data for the user with the weakest radio link, i.e. the lowest channel gain, may be transmitted with the greatest power. Transmit power for the remaining users in the superposition group is assigned in order of descending channel gain so symbols for the user with the greatest channel gain are transmitted with the lowest transmit power. Upon receiving the signal, a UE can successively detect, and optionally decode, then cancel data signals transmitted at a higher power, i.e. cancel signals of interfering users, to reveal its own data.
The use of OFDMA with superposition/cancellation, i.e. NOMA, has been proposed as a radio access technology for future radio access systems. Such a system could continue to encode data as a set of complex symbols as is done in conventional OFDMA systems and achieve link adaptation with adaptive modulation and coding (AMC), similar to conventional LTE based systems. NOMA could be incorporated through the use of multi-user power allocation as described above.
NOMA has the potential to significantly improve the spectral efficiency of a wireless system and provide improved throughput and QoE. However, these benefits come at the cost of increases in receiver complexity and increased signaling requirements between the base station and the UE.
In conventional LTE based systems, signaling or control information is sent from a base station to a UE over a physical downlink control channel (PDCCH). The PDCCH may be used to send downlink control information (DCI), such as the parameters necessary to decode data being sent to the UE over the physical downlink shared channel (PDSCH), from a base station to the UE. When desired this decoding information can be sent dynamically within each transmission time interval. In conventional systems, such as LTE, there are several DCI formats available and are selected as necessary to keep DCI transmission overhead reasonably small so they may be transmitted dynamically or on a per transmission time interval (TTI) bases. Some of the more common information carried over the PDCCH using the various DCI formats is listed below:                Resource block assignment: a resource block (RB) is the smallest individually schedulable portion of the radio resources and comprises a plurality of resource elements. A resource element (RE) is the smallest usable portion of the radio spectrum and has dimensions of time and frequency. The resource block assignment indicates the position of RE within the RB allocated to the particular UE.        Modulation and Coding Scheme (MCS): the MCS is often sent as an index value, which may be referred to as a channel quality indicator (CQI) index, where each index value is assigned to a predetermined modulation scheme, such as 16 symbol quadrature amplitude modulation (16QAM) and a code rate which defines the number of redundancy bits used during channel coding. In many conventional wireless systems binary data is transmitted by converting or modulating it as a set of complex symbols selected form a modulation alphabet where each symbol is assigned a predefined bit sequence. These symbols are then transmitted via the wireless interface to a receiver that detects each symbol and converts it back to its binary sequence.        New Data Indicator (NDI): used for hybrid automatic repeat requests (HARQ).        Precoding Matrix Indicator (PMI) and Rank Index (RI): used to select the precoding matrix and specify the rank for spatial multiplexing with multiple antennas.The CQI index may be derived by the UE and reported back to the base station based on for example measurements of the downlink reference signals. The CQI reporting is configured by the base station through radio resource control (RRC) signaling sent to a UE when it first connects to a base station.        
Combining conventional orthogonal transmission technologies with NOMA techniques can significantly increase the amount of signaling necessary between a base station and the UE to which it is sending data. Adding this signaling to existing DCI structures can burden a communication system with a large amount of signaling thereby reducing the radio resources available for data transmission and reducing the data throughput. Thus there is a need for improved methods and apparatus for signaling NOMA information that avoid adversely impacting data throughput.